I was working on a few childern models (unrigged) inspired from Fallout and Skyrim but I never did complete them. A big concern would be that people would want to use them in their games whereas they could actually kill them. That's a bit disturbing of a thought.
I could easily script it so that if the cross hair ever went over them, the player simply wouldn't beable to fire and their weapon would automatically holster, but then again whose to say someone just wont change the script?
I say your best bet is to maybe use a decal of an image of a child for the effect you may be looking for. That's what they do in most games where a creepy child appears. Its simply an image that appears and fades out/disappears.